My deep understanding of vitamin D began during my doctoral career. As I became a mom more than a year ago, I have been exploring and learning a lot in the process of raising my baby, and I understand more and more how other parents struggle with this issue. Although nutrition follow-up and education in China is still in its infancy, the public’s awareness of calcium supplementation far exceeds that of other nutrients. Is my child deficient in calcium? Is this disease caused by calcium deficiency? This is a common question asked by many pediatric orthopedic patients. Is it or is it not? If you want an accurate answer, a blood test is required. There are two tests that are usually done, one is blood calcium, phosphorus, magnesium + alkaline phosphatase, and the other is blood 25(OH)D. The first one, everyone understands; the latter one, not so much. Does the blood calcium result reflect the calcium deficiency of the child? No! The stability of calcium ion concentration is an important basis for normal physiological activities of the body, and the body has many kinds of mechanisms to maintain the stability of blood calcium. When the calcium ion concentration in the blood decreases, the absorption of calcium from food in the small intestine increases, and calcium is released from the bones to replace the missing calcium ions in the blood. Therefore, reduced calcium ions are often seen in patients with severe systemic disease, rather than the usual concern of bone calcium deficiency. What is the cause of what is commonly referred to as calcium deficiency? For the general public, the symptoms of so-called “calcium deficiency” are often caused by vitamin D deficiency. The function of vitamin D is to increase the absorption of calcium in the intestine and promote the transfer of calcium to the bones, which is the reason why vitamin D can “strengthen bones”. Vitamin D deficiency is very common in China, especially in the north, where more than two-thirds of the patients seen in outpatient clinics are vitamin D deficient or lacking. Vitamin D deficiency not only causes night terrors, sweating, square cranium and other symptoms of rickets, but also weakens the child’s resistance and makes the child susceptible to infectious and immune diseases. Growing pains in children during the rapid growth period are also often associated with vitamin D deficiency. In adults, especially menopausal women, vitamin D deficiency significantly increases the incidence of fractures, and they need more vitamin D than normal to resist hormone-induced bone calcium loss and maintain bone stability. The American Academy of Pediatrics recommends supplementation with 400 IU of vitamin D daily from birth until 2 years of age, with this end age being continually postponed, and some even recommend supplementation until adulthood. I personally strongly support this recommendation and have been consistent in doing so. In our country, children and adolescents have heavy school workloads and insignificant daylight hours, and a significant number of children who come to our department have abysmally low vitamin D levels, so I personally recommend that this supplementation program be continued indefinitely. There is a wide range of D supplements on the market and no one can possibly try them all. So for parents with a phobia of choice, there is no need to put too much thought into selecting a brand. The regular manufacturers, the dose of adequate can be. Good or bad, it depends on the efficacy! If the child does have a vitamin D deficiency, then the supplement should be reviewed after a month or two. If the brand is good, we will know when the time comes …… Is there a relationship between D deficiency and fractures? Yes, it does! Does it have anything to do with leg pain? Yes! Does it have anything to do with legs not being straight? Yes! Does it have anything to do with ……? It has to do with almost every disease you can think of. Therefore, parents who are concerned about their children’s “calcium deficiency” should pay attention to the role of vitamin D, actively test for deficiencies, and take targeted supplements to take care of their children’s bone health with a positive scientific attitude.